1. Field of the Invention
The present invention relates to a monolithic ceramic substrate manufactured by a process that does not cause shrinkage of the monolithic ceramic substrate and a method for making the monolithic ceramic substrate that does not shrink along the principal plane during sintering.
2. Description of the Related Art
As disclosed in Japanese Unexamined Patent Application Publication No. 7-86743, a monolithic ceramic substrate is manufactured by a process which does not cause shrinkage of the monolithic ceramic substrate. More specifically, a laminate including a plurality of ceramic layers made of glass ceramic that sinters at low temperatures is disposed between two inorganic constraining layers that do not sinter at a sintering temperature for the ceramic layers, this composite is sintered, and then the constraining layers are removed.
Conductor patterns are disposed at desired interfaces between the ceramic layers and the constraining layers and on outer surfaces of the laminate. These conductor patterns are formed by etching a metal foil that does not shrink at the sintering temperature for the ceramic layers.
This method, however, has the following problems. The thin-film conductor is a dense film. Thus, the thin-film conductor does not include interstices for releasing gas generated by decomposition of a binder component in the green ceramic sheets during sintering in a pattern having a large occupancy area, such as a ground electrode pattern. As a result, voids and residual carbon, which greatly reduce the reliability of the monolithic ceramic substrate, tend to remain in the vicinity of the thin-film conductors.
Since the thin-film conductor has low mechanical strength, the conductor must have an increased thickness. Thus, the thin-film conductor does not shrink during sintering, and as a result, the soft green ceramic sheets are compacted. The soft green ceramic sheets are dense at patterns having large occupancy areas but are not dense at patterns having small occupancy areas, for example, inductor electrodes and signal lines, resulting in a large difference in the density in the green ceramic sheet. As a result, the internal stress in a circuit board including many patterns having large occupancy areas is greatly increased, which causes deformation or cracking.